1. Field of the Invention
The present invention relates to closures for metal beverage containers. More particularly the present invention relates to container closures having increased strength.
2. Description of the Prior Art
Metal beverage containers ae a very competitive product in the packaging industry since the annual production of these containers is well over 70 billion per year in the United States alone. Even a small reduction in the thickness of the metal used in the container closure can result in savings of millions of dollars annually.
The closures for the containers typically include a center panel that is generally planar but domed upwardly, a center-panel ring that is disposed annularly around the center panel and that curves downwardly therefrom, an inner leg that projects donwardly from the center-panel ring, a curved connecting portion that connects to the inner leg distal from the center-panel ring, an outer leg that connects to the curved connecting portion and that extends upwardly, and an outer curl that is used for double seaming to the container.
One of the limitations in the strength of a container of this type is the internal pressure at which buckling of the closure occurs. Buckling refers to a permanent and objectionable deformation of the closure, including the inner leg, the outer leg, and the center panel, in which circular uniformity of the closure is destroyed by fluid pressure that is exerted inside the closure.
Various attempts have been made to increase the buckling pressure of container closures; and these attempts are represented by issued patents which are discussed below.
Gedde, in U.S. Pat. No. 3,774,801, teaches complex doming of the center panel that includes a plurality of circumferentially-uniform steps as a method of increasing the buckling pressure of the closure.
Khoury, in U.S. Pat. No. 3,441,170, teaches coining of the inside of the center-panel ring as a method of allowing the center panel to dome under pressure without the doming exerting a full buckling force on the inner and outer legs of the closure. The inventor states that the coined area functions as a hinge.
Jordan, in U.S. Pat. No. 4,031,837, teaches increasing the buckling pressure by reforming the closure with a reduced radius in the curved-connecting portion that interconnects the inner and outer legs, by increasing the angle of the inner leg to substantially vertical, and by moving the curved-connecting portion downwardly from the center panel.
Kraska, in U.S. Pat. Nos. 4,217,843 and 4,448,322, teaches a reforming operation in whch the outer leg is positioned more nearly vertical, the inside radius of the center-panel ring is reduced, and the inside radius of the center-panel ring is coined to produce doming of the center panel.
Some doming of the center panel has been found to increase the buckling pressure of the containers because it eliminates any excess metal that results from scoring for opening. The presence of excess metal allows an uneven distribution of stress in the center panel, so that buckling results at lower internal pressures.
The prior art includes U.S. Pat. Nos. 4,434,641 and 4,577,774, both of common ownership to the present invention. In these patents, Nguyen teaches coining the convex outside surface of the center-panel ring to increase the buckling pressure of the container closures.
The prior art also includes patent application Ser. No. 06/075,384 of common ownership to the present invention. This patent application teaches increasing the buckling pressure of container closures by double coining, or by curvilinear coining, the convex outer surface of the center-panel ring and/or portions of the center panel or the inner leg.
Coining is a local deformation, by cold-working, of metal by reduction of thickness in a specified and limited, or predetermined, area through a mechanical pressing operation. Cold-working may, or may not, include a reduction in material thickness; but coining always includes a reduction in thickness of some localized area of the material, even through this reduction in thickness may be slight.
Coining a container closure produces compression doming of the center panel. Optionally, this doming can be limited by providing a hold-down pad, as taught by Nguyen in the aforesaid prior art patents.